One of the problems when playing back media to a single user or small
group of users is choosing a multicast address for the playback. This
problem does not arise when doing playback to a conference because
conferences are usually on well known or well defined addresses. In
choosing an address for a user there must be no clash with existing, in-use
multicast addresses.

As well as uniqueness of the address it has to be decided who makes
that decision. There seems to be four choices for who does this:

1.

the user

2.

a client

3.

the server

4.

an external service

For the first three choices it is hard to determine how they can guarantee
some uniqeness given they have no mechanism for deciding which addresses
are already in use, and therefore which are free to use. The most
appealing choice is for some external service to make the decision. This
service can be used by all systems that require a new multicast address for
their purposes. Although such a service does not currently exist, it is in
the design phase. The session directory tool [#!sdp!#] already has an
intelligent mechanism for choosing new addresses for conferences started
under its auspices, but is not usable by other systems as it is standalone.
Work described in chapter seven will culminate in the
definition of a session description protocol, and should lead to the
implementation of a server that does allocate new multicast addresses on
demand. Once this server has been implemented any program can get unique multicast addresses.